Intertwined is the most appropriate description for the recent relationship between the main contractor and its foundation contractor creating a site for prestigious city centre flats cut deep into the steep sloped banks of Newcastle's River Tyne.
'For over 12 weeks we did little else but repeatedly provide zig-zag access ramps for Bachy Soletanche's piling rig as it worked its way down the hillside, ' explains Ken Dyer, site manager for main contractor PJ Carey.'Without total and close cooperation between us, this job would certainly have ground to a halt.' To the client, City Loft Developments, location was the key word in siting the three, up to seven storey high blocks of luxury apartments.
And what better location than close to Newcastle's first £1M flat, overlooking the north east England city's famous cluster of river bridges.
To provide such a location, a large 20m high wedge shaped chunk of the northern river bank has to be removed. Bachy Soletanche spent much of last year forming a 115m long multilevel network of contiguous piled retaining walls to create stepped, level sites for the three apartment blocks with their integral underground car park.
'The plan was to drive the piling rig in at the top of the hill, work down forming the walls, and drive out at the bottom, ' recalls Bachy Soletanche contracts manager Rob Howarth.
'But it did not quite turn out like that, and we had to bring in a crane to recover the trapped rig half way down.' The hillside, with its 30infinity slope, offered a congested location both above and below ground.
Barely 45m wide, the site boasts an important flight of 16th century steps up the middle; the remains of a medieval timber wharf at its base;
an electricity substation; and two major live sewers running right through the site and down the bank.
Impose on these obstructions some 210 continuous flight auger piles, bored a maximum 22m deep to create a large retaining wall and it is little wonder engineers describe the job as the most complex, challenging project they have yet worked on.
'It is a three dimensional site that has proved much more complicated in practice than any of us realised, ' says Howarth, adding that the seven different piling levels needed, and 15 rig locations, were more than double the numbers originally planned.
Central to Bachy Soletanche's six month, £600,000 contract has been the formation of two separate retaining walls. An upper 30m long wall, of 13m deep CFA piles bored across the top of the hillside, has been excavated to a depth of 6m to provide a retained, level site for the upper of the three apartment blocks.
Further down the slope, and along the front edge of this upper section, a larger, more complex shaped 85m run of 600mm diameter, 22m deep piles. More than 30 temporary ground anchors have been drilled in two rows into its front face and each 15m long, multi- strand Dywidag tendon stressed to a maximum 750kN.
This 110 pile wall is now being exposed to a depth of 15.6m as the lower site is excavated for an underground car park and the two remaining blocks of flats.
Cobbles up to 200mm diameter in the slope's glacial clay meant Bachy Soletanche's Solmec CM700 powerful CFA rig would be the most suitable, and it was accepted this large 75t machine would present a manoeuvring challenge as it negotiated the slope. Carey's main job at this stage was to keep the rig moving by repeatedly building ramps across and down the hillside.
Ground conditions dictated a four pile gap between bores, and the awkward site geometry sometimes meant that the rig could complete only one pile a day. Half way down the slope, with the machine wedged into a corner, and no more piling positions available, the site team had a rethink.
The most economic solution was to remove the rig altogether from the site for several weeks while Carey provided new piling areas.
But the practicality of plucking the stranded machine off the hillside was less straightforward.
There was no room on the road beneath to bring in the vast 500t crane that would have been needed to provide the height and reach required to lift out the complete rig. Instead a smaller 200t mobile version arrived and the rig was stripped down to just its undercarriage and tracks.
Now weighing in at less than 45t, this chassis unit could tackle steeper slopes and drove itself down the remaining hillside.
'But it still took us four days to complete a normally four hour operation, ' Howarth says.
Six weeks later, Bachy Soletanche brought in a much smaller Solmec CM48 rig. Its 8.7tm torque was just half that of the larger machine but so was its weight.
The gamble paid off as the rig still coped with the cobbled ground and lengthy piles, while proving much more manoeuvrable around site.
With piling complete on schedule before Christmas last year, and the last of the 33 ground anchors now stressed, attention has turned to exposure of the retaining wall.
Howarth's confidence that the excavated piles will 'look good' could be tested to the full as the 22m high hillside structure will be very visible to motorists and train passengers crossing any of Newcastle's five nearby river bridges.